1. FIELD OF THE INVENTION:
This invention relates to apparatus for determining the quantity and/or other physical parameters of particles, images thereof, or image patterns lying within the range of a scanning device and, more particularly, to apparatus for making such determinations when the particles are of arbitrary size and shape or are randomly or regularly oriented, or any combination of these.
2. DESCRIPTION OF THE PRIOR ART:
Detection and analysis of particulate materials is a necessity spanning a broad spectrum of scientific, engineering and industrial disciplines. Various types of systems have been either proposed or actually used in measuring the quantity and/or physical parameters of a group of regularly or randomly oriented particles of various size and shape. There are two basic approaches to the analysis of any particle population; indirect measurements of secondary effects manifested by the particles or direct measurement of particle parameters via an optical or electron microscope.
The indirect techniques employed permit relatively quick measurement of the particle population under study. This approach is typified by methods such as sedimentation of particles. A suspension is allowed to settle by gravity or centrifugation whereupon the optical clarity of the mixture is measured as a function of time and/or position. By then assuming a number of factors, such as particle density, hydraulic flow resistance, multiparticle interactions and agglomeration characteristics, one can infer particle parameter information.
Light scattering is another analog technique used for measuring both discrete particles and aggregates. It assumes a known, invariant relationship between the diameter of a particle and the proportion of incident light scattered by that particle. Individual scintillations caused by a particle entering the scattering zone of the measuring apparatus are then detected. The detected information can be gated into various sizes categories to yield instantaneous results or transformed into a permanent record. In these instruments, certain factors such as the effects produced by particle shape, color, coincidence and refractive index must be ignored.
Because the various analog techniques only make indirect measurement of the particles under study, and because certain factors are assumed or ignored, or both, the results obtained are frequently questionable. Thus, only the grossest assessment of actual particle characteristics can be derived. Precision measurement is obviously not possible in most cases and, in addition, many particle parameters cannot be obtained using indirect analog techniques.
Microscopic measurement of a particle sample, on the other hand, permits the measurement determination of particle parameters to any degree of precision desired. Further, one can measure any number of different particle parameters using this technique. An extremely wide size range of particles can be accommodated through microscopic examination and, most importantly, this technique furnishes the investigator with a direct and unambiguous measure of the particle population under study. This ability to obtain precise, detailed information is a paramount advantage in particle analysis. However, manual microscopic methods do have significant disadvantages. Measurement with the microscope is a slow process and was, until the present time, considered particularly unsuitable for use in conjunction with automated processes. A comparatively long measurement interval is required to achieve statistically valid results. A relatively high degree of operator skill and technical knowledge are necessary for accurate results. In addition, operator fatigue is a limiting factor in repetitive sampling. Finally, the data obtained by using the microscope must be mathematically transformed into some meaningful set of measurements. All of these disadvantages combine to limit the number of samples which can be processed by this manual technique.
These problems have been eased somewhat by improvements designed to fascillitate particle measurement. Recently, particle measuring systems have come into use wherein a television camera is employed to pick up the microscopic image for eventual projection on a monitor where particle measurements are made on the particle image directly or indirectly by associated equipment. Generally, these prior art systems used a television camera or scanning device which was played over a discrete region containing the particles of interest. Interception of a particle in the path of the scanning device produced an electrical signal which was operated on by the remainder of the system to yield the desired particle parameter.
In some systems, the number of signals or intercepts was totaled in a simple counter to yield a signal representative of the total intercept count within the region being scanned. In other systems, measurements were made of the interval between intercept signals or their frequency in order to derive dimensional information concerning a particle. Still other systems employed two beams or scanning devices moving along adjacent scan lines, comparing the electrical signals derived therefrom in order to separate the signal as belonging to one or another particle. In still other systems, only one scanning device was used, but no intelligence was gathered until the results from two successive scans were compared. Finally, there were hybrid prior art particle measuring systems which were variations of the above-noted systems with an occasional modification incorporated therein to allow an unusual measurement to be made or an unusually shaped particle to be measured.
The availability of such devices lessened the need to resort to the accurate but time consuming and laborious microscopic manual methods previously employed. Unfortunately, these prior art systems were usually intended or designed to be particularly suitable for measuring only one specific particle parameter. While these systems worked well enough, they tended to be, as "laboratory" type instruments frequently are, overly sophisticated and too expensive for their intended purpose. This disadvantage, when coupled with the single-purpose approach of the designer, rendered these early particle measuring systems unsuitable for commercial use. In addition, these prior art systems, because of burgeoning academic and industrial research efforts, were also proving unsatisfactory in the very places which originated them.
In time, systems which were capable of making more than one type of measurement became available. Typically, these systems could determine the quantity of particles scanned, compute a particle's maximum chord as measured in the direction of scan or measure the total area of all particles lying within a discrete region. Generally, these were the only measurements which could be made, unless the slower manual microscopic methods or a specialized instrument were employed. As previously mentioned, indirect techniques proved unsatisfactory for all but a few exceptions because of their inherent inaccuracies.
In determining the count or number of particles within the region, the scanning device produced a sweep thereover causing the generation of an electrical signal whenever a particle was encountered. Another signal of a differing level, or no signal at all, was generated when a particle was not intercepted along a line of scan. Such devices worked well enough from an accuracy standpoint where the particles were generally regular in shape. However, this condition was the exception rather than the rule. In the overwhelming majority of the cases, the particles to be measured were of irregular shape having reentrant profiles, contained holes or voids or were randomly oriented. As a result, the particle count or measurement obtained was all too often inaccurate, while the partial solution provided by the slower manual graphical methods was unsatisfactory from both a time and accuracy viewpoint.
The same poor results were obtained where the maximum chord of a particle or the total particle area, rather than a total count, was desired. Firstly, complex particle periphery caused difficulty in obtaining maximum chord or area measurements. Secondly, it was only possible to obtain the maximum chord of a particle in the direction of scan. Obviously, this limitation proved frustrating when the maximum chord lay in other than the direction of scan as is the case in the majority of instances. Thirdly, when present, voids or holes in the particles further compounded the problems of obtaining accurate area measurements. Fourthly, only a limited number of simple measurements could be made. Again, the time consuming and laborious manual microscopic methods provided only a partial solution or alternative. The overall result was that the need for accurate, rapid and versatile particle or image measuring systems was left largely unsatisfied.